At an earlier hearing he pleaded guilty to touching two 15-year-old girls in their own homes while he was there to coach them in mathematics.

Khan, who was described in court as a "distinguished and successful" mathematician, made national headlines in 1997 when his 13-year-old daughter won a place to study maths in Oxford University.

Yesterday, the court heard Khan had given up work with the Ministry of Defence in the 1980s to home educate his five children.

Khan developed such an impressive method of teaching that governments throughout the world had asked him to share his skills with them.

In 2002 he had started advertising his services as a home tutor, under the name Farooq Yusof.

Prosecuting, Blondel Thompson said in May 2007 Khan arrived at one of the victim's home for a maths lesson.

The girl was alone with Khan for the lesson while her parents were out and her sister was upstairs.

Khan started whispering in her ear and kissing her hair and cheek. He also touched her breast and told her she was a genius and pretty before putting her hand on his upper thigh.

The girl eventually managed to get away and run to her sister, who contacted their mother, and got rid of Khan. The following day a teacher noticed the girl was upset and informed the police about what had happened.

While being interviewed, it transpired that two years earlier Khan had been at the home of another 15-year-old girl when he started kissing the pupil and put his hand on her thigh.

Defending, Tim Hannam said Khan felt great remorse and shame for what he had done and added his client had not become a tutor in the hope of grooming young girls.

"He's been back in prison for over five months and knows there'll be no more teaching and any hope he had of gaining an income from the teaching method he had developed to a high degree of success is lost to him. His reputation is destroyed," he said.

He said Khan had suffered criticism in the media after his daughter ran away from university and he had also been involved in three car accidents which left him with post-traumatic stress and depression.

Sentencing him to 18 months in prison, minus the 161 days he has already served, Judge Eccles said Khan had abused his position of trust.

"This was not a one-off incident by a man in distress but indicative of a deep-seated and inappropriate sexual interest in teenage girls," he said.

He added: "As a maths teacher you used your professional standing to obtain the trust of parents and their children."

The court heard Khan has already served a three-year jail sentence after being convicted of dishonesty in 1992.